As is known, engineers and scientists strive to develop wireless communication systems that achieve high data rates at relatively low costs. High data rates are often attainable by design but cannot be implemented because costs are prohibitive.
In fourth-generation wireless systems (known as “4G” systems), capacities of 1 Gbps for a local area and 10 Mbps for wide area coverage have been envisioned. However, even with very wide bandwidths on the order of 100 MHz, the spectral efficiencies needed for these capacities are extremely high. One way of achieving these spectral efficiencies is the use of multiple antennas to transmit and receive multiple simultaneous data streams, i.e. MIMO (Multiple Input Multiple Output) transmissions. Also, due to lack of free spectrum in the lower frequencies, carrier frequencies around 5 GHz have been discussed. Even if the spectral efficiency requirements can be fulfilled, though, such systems would still have short ranges, due to the attenuation of high frequency signals. This combined with the limitations on transmit power and the requirements for high data rates, make communication cell ranges small, thus increasing the overall cost of the network. To combat the problem of small cell ranges, researchers have proposed the use of a multi-hop network architecture. For example, Otsu, Aburakawa, and Yamao discuss multi-hop systems in “Multi-hop wireless link system for new generation mobile radio access networks,” IEICE Trans. Comm., Vol. E85-B, No. 8, Aug. 2002. Multi-hop architecture refers to the use of relay stations, without connection to fixed network, in addition to the normal base stations. Provided that the relays can be made considerably cheaper than normal base stations, such an arrangement may help with the range problem.
Even assuming that such multi-hop relays are able to use higher order modulations than the mobile stations, the combination of very high data rates and wide area coverage is still a problem. Multiple antennas can be used at the relays to increase the range to the next relay or base station or to increase the data rate, but to achieve both simultaneously is not an easy task. Capacity of the channel gives the maximum data rates which are possible to transmit reliably over the channel. However, in practical systems, a discrete set of modulations are used, and the maximum data rate can be limited by the modulations rather than the capacity. In particular, higher order modulations, such as M-QAM (quadrature amplitude modulation), where the cardinality of the modulation M is high, may not be used in practical systems. As the cardinality of the modulation increases the transmission gets less reliable. This can be compensated either by increasing the transmit power or by using a more complex receiver. But since the transmit power is limited and the cost of implementation of the receiver needs to be kept as low as possible, there is a strict upper limit for the cardinality of the modulation.
As an example of the contradicting requirements of high data rate and long range, consider a user equipment (UE) device having four transmit antennas in propagation conditions which allow for spatial multiplexing with three parallel streams. Assuming that the highest order modulation possible is 16-QAM, i.e. 4 bits/stream, the total data rate is 12 bits per channel use. Between relay and base stations, higher order modulations may be possible, such as a 128-QAM transmission. However, 128-QAM corresponds to only 7 bits/stream, meaning that at least two streams would be needed to transmit the same information between the relay and base station. Therefore, such a relay topology would require multiple stream transmission between all relay stations in a cell and the base station. Such a topology may not be possible in all environments, or the range will suffer. High rate multiple input, multiple output (MIMO) transmission requires rich scattering environments to work well, but in these kinds of environments the attenuation is strong and consequently the range is small.
U.S. Pat. No. 5,345,599 describes a distributed transmission, directive reception system that applies macroscopically distributed MIMO (multiple input, multiple output) transmission using wireline links to distribute the streams to the macroscopically separated transmitters. However, the spatially distributed transceivers in this system are connected with wireline to the base station, making it very expensive to deploy.
U.S. Pat. No. 6,067,290 describes a system with distributed transmission of multi-streams to/from one user equipment (UE) to multiple base stations (BSs). However, in such a system, coverage becomes a problem as deploying wired BSs is very expensive. In addition, macro diversity and multiplexing in this system requires extensive signaling between BSs over the core network.
Thus, there is a need for improved high data rate transmission in multiple input, multiple output (MIMO)networks. Further, there is a need for a wireless multi-hop system with macroscopic multiplexing. Even further, there is a need to achieve high data rates in wireless communication systems at relatively low costs.